“All the girls in our shared dressing room burst into applause,” the actress recalls. Barnes, meanwhile, grew pale and considered various ‘I forgot something in my car’ style emergency exits. “I’d never even done the show all the way through before,” explains the 26 year-old, who – with her heart-shaped face, impish smile and little curvy body – could pass herself off as Smith’s sister.

“I’d never even done the show all the way through before,” explains the Hampshire-born 26 year-old: the gentle voice, face and demeanour in wild contrast to her mesmerising on-stage exuberance. “But as Sheridan’s understudy it was my job, so I went out there then and gave it my all – just as I’m giving it my all every night now – and I suppose I hoped that would be good enough.”

Not only was Miss Barnes’s ‘all’ good enough to placate the grumbling ticketholders who had paid over the odds to see Smith play a role immortalised by Barbra Streisand in the 1964 Broadway production and subsequent film, but the critics were stunned.

“Remember the name,” warned The Stage, which happened to be there that first night that Barnes covered Smith, while the show was still at the fringe theatre of the Menier Chocolate Factory. Borrowing Julian Marsh’s famous declaration to Peggy Sawyer in 42nd Street, the writer declared: “She may have gone out an understudy but she came back a star.”

Quite a feat for the daughter of a council officer and a former aviation company director from Ringwood who got her first theatre role at 18. Barnes had been about to take a job at Waitrose when the Broadway director Michael Mayer gave her a small part in a production of Spring Awakenings in London’s West End.

There Barnes has worked steadily in the back line for the past eight years – with the occasional TV appearance in the likes of ITV’s Doc Martin – until once again called upon by Mayer, this time to understudy Smith in Funny Girl. The rest is history (in the making).

And although Barnes won’t discuss either the nature of Smith’s problems or exactly how long she’ll be standing in for her (“I really don’t know”), she is the first to admit that her fairy tale story is bittersweet: her success only being possible thanks to Smith’s misfortunes.

Struggling to deal with news that her father was diagnosed with the same cancer that killed her brother a decade ago, the Cilla star has been indisposed on several occasions since March – with one production halted 15 minutes into the play “due to technical difficulties” and audience members claiming Smith was actually drunk and slurring her words.

“This is absolutely the pinnacle of my career,” Barnes assures me, still clearly dizzied by the turn of events. “Because for eight years I’ve been losing out to people who had bigger profiles than me and to be here now, and getting standing ovations is amazing.

But because Sheridan’s a friend and we’re a family, it’s never going to feel whole without her here. When I’m on stage I don’t have time to consider it but backstage before and after the show there is this absence. I miss her and worry about her and I hope that she’s alright.”

Ever since Barnes took on the role of Sheridan’s dresser in the show – which started at the Menier back in December before moving to the Savoy Theatre last month – the two have grown close. “We’re actually very alike even though we’re from opposite sides of the country and she’s got eight years on me.

Neither of us had formal training and we’re both people people – but most importantly we have the same heart and the same soul.” Barnes and Smith have been exchanging text messages since the older actress took a leave of absence, she goes on, “and Sheridan’s been so supportive of me. ‘You give this a go and it will do good things for you’, she told me. ‘But be brave and make the part your own’.”

Sheridan Smith in the role of Fanny Brice in 'Funny Girl'Credit:
Marc Brenner

But however inebriating the accolades and the autograph hunters waiting outside the stage door may be (“I’d never signed an autograph before,” she chuckles) Barnes is now acutely aware of the pressures Smith was under.

“It’s physically and emotionally exhausting,” admits the actress, who says she is having trouble sleeping and has resorted to using a pen and paper to converse with her boyfriend at home in order to safeguard her voice. “Every time I reach the end of a performance I wonder whether I can do it again the next day.”

Were she grappling with private pressures like Smith, would she be able to keep up the momentum? Barnes ponders this a while. “Actually I have used private grief in the past. When I stood in at the Menier it was just after my granddad passed away and there’s a scene where Fanny says goodbye to a certain character that would always make me emotional, because things are said that were similar to my last conversation with him.

Natasha Barnes has been compared to Judy GarlandCredit:
Manuel Harlan

Then when it came to the funeral a week later, it was the strangest thing: I didn’t cry. I couldn’t because I’d done it eight times already, and I realised that it could be a dangerous trap to fall into. Because once you get into the habit of using your own heart for work, you can very quickly forget how to act without it, and then you exhaust yourself.”

What helps abate the pressures of performing every night – and raised more than a few knowing laughs on the night I saw Barnes in action – are the parallels between her life story and Fanny Brice’s. “It sometimes feels as though the words coming out of my mouth aren’t scripted but mine,” she smiles.

Because just like Fanny I’m not here to look pretty. Her whole career was banging down doors despite not being tall and leggy and I think in a way it makes it even more satisfying to have to fight to get in the door.

Also when Fanny says ‘I’d better get used to the publicity’ that sums up my life perfectly at the moment.” And although I get the feeling Barnes won’t assume the mantle of celebrity quite as readily as so many would in her position, she’s already developed the thicker skin needed to navigate social media.

“Because of course there have been a few unpleasant Tweets. And actually I understand it: you pledge allegiance to a performer and I’ve certainly never seen a more beautiful performer than Sheridan. But I have real faith in the audience, and I honestly feel that no matter how disappointed some people might feel that they booked to see her and got me, they will find a way of enjoying the show.”

There was, however, one Tweet that hit a nerve. “Someone said: ‘tell Natasha Barnes to stop with the fake tears every night’ – and my goodness they’re far from fake. When I see all those people standing up for me at the end of a show, those are tears of absolute shock and joy, and I can’t help but cry them every night.”

So won’t it be soul crushing to go back to being Fanny Brice’s dresser, once Smith returns – whenever that may be? “Nope,” insists Barnes, and it’s a testament to her acting skills that she keeps that smile so wide, and so bright. “I’ve done it before and I’ll do it again. And maybe one day I will have something that’s all mine, but right now my job is to support the company and to support Sheridan.”

I can only hope that the actress who understudies Barnes as Brice’s dresser feels the same. “Do you know what?” giggles Barnes. “She’s this ridiculously tall and beautiful Bond-style girl. So every time I look at her playing the part, I have trouble taking it all in.” A common problem for any actress who has been understudied, but Barnes may just have to get used to it.